1. Field of the Invention
The invention relates to probes and probe bodies for measuring a desired parameter, such as capacitance. More particularly, the invention relates to a capacitance probe and probe body of the type used for measuring capacitance to detect the level of liquid in a pressure vessel, and still more particularly, to such a probe that resists leakage and, in one embodiment, is explosionproof.
2. Description of the Related Art
In industrial metrology, capacitance commonly is used to determine the level of liquid in vessels, particularly in closed vessels. One method of measuring capacitance is through the use of a concentric-type plate capacitor. A concentric-type plate capacitor utilizes a conductive probe as the center plate of a coaxial capacitor. This conductive probe is usually a cylindrical metal rod (center or probe rod) that is insulated to measure conductive liquid levels. The vessel walls typically form the second plate of the capacitor while the liquid to be measured is the dielectric. The capacitance of a concentric-type plate capacitor is given by the equation: ##EQU1## EQU where C=capacitance;
.epsilon.=dielectric constant (1.000590 for air); PA1 L=length of concentric immersion by the probe rod; PA1 r.sub.0 =radius of the inside of the outer wall of the vessel; and
r.sub.1 =radius of the metallic probe rod.
The presence of material in the vessel creates a concentric-type plate capacitor with the liquid serving as the dielectric. Therefore, if one knows the dielectric constant of the material in the vessel, by measuring the capacitance, C, it is possible to determine the level of material in the vessel. The probe is connected via suitable electrical connections to capacitance-measuring circuitry. In measuring the capacitance, a lower intrinsic capacitance of the capacitance probe itself is desirable in order to more accurately gauge changes in the overall capacitance that are caused by changes in the liquid level.
Typical capacitance probes use metallic mounting glands to mount the probe rod to the vessel and to the electronics housing. The internal portion of the mounting gland contains: (1) a pressure sealing system; and (2) a probe rod retaining system. The probe rod itself is typically a metallic rod sheathed in polytetrafluoroethylene (PTFE) or other suitable polymer. The probe rod must pass through the metallic mounting gland with no metallic contact.
Referring to FIG. 1, an existing capacitance probe includes a packing gland 10 that is threaded into a mounting gland 12. A center rod or probe rod 14 extends from an electronics housing (not shown) through metal packing gland 10 and mounting gland 12 and into a vessel (not shown). Center rod 14 comprises an upper rod 13 that is threaded into a lower rod 15. Because mounting gland 12 is electrically connected to the vessel walls, center rod 14 must be electrically insulated from mounting gland 12 to achieve an accurate capacitance reading. Therefore, a lower sheath 16 surrounds lower rod 15 and an upper sheath 17 surrounds upper rod 13. Sheaths 16 and 17 are typically composed of PTFE although other polymers may be used. A solid PTFE cap or plug 19 insulates the end of lower rod 15 and is joined to lower sheath 16 thermally.
Housed inside mounting gland 12 is a packing preform 18 that is compressed against lower sheath 16 to form a pressure seal. Packing preform 18 comprises an upper portion 20 and a lower portion 22. Threading on packing gland 10 and mounting gland 12 allows packing gland 10 to be torqued and tightened against packing preform 18 via a PTFE washer 26 and a metallic washer 28. When packing gland 10 is tightened, lower portion 22 of packing preform 18 applies pressure against lower sheath 16 to create a radial pressure seal.
PTFE and other polymers, however, cold flow under pressure. Thus, lower sheath 16 cold flows in the area that is engaged with lower portion 22 of packing preform 18. This cold flow reduces the pressure seal and allows leakage. Deformation also occurs above and below metallic washer 24 in lower sheath 16 and upper sheath 17.
These deformations require periodic retorquing of packing gland 10 to maintain compression. As this retorquing process is repeated, lower sheath 16 flows to the point of separation and the seal fails.
A second difficulty encountered with capacitance probes is the maintenance of center rod 14 against hydrostatic end forces. To solve this problem, existing capacitance probes include one or more metallic washers 24 placed in a groove at a threaded junction point in center rod 14 located within mounting gland 12. Packing preform 18 is made as two separate pieces rather than one in order to facilitate encapsulation of metallic washer 24. Hydrostatic pressure acts against metallic washer 24 in an upward direction forcing it against upper portion 20 of packing preform 18. Although center rod 14 is contained because the outer diameter of metallic washer 24 is greater than the inner diameter of packing gland 10, cold flow of upper portion 20 may occur. Thus, a capacitance probe capable of reducing or eliminating cold flow of the capacitance probe's PTFE is desirable.
In addition, a need exists for a capacitance probe and probe body that is explosionproof under the standards promulgated by the National Fire Protection Association in The National Electrical Code.RTM. Handbook (5th ed.).
Finally, a probe and mounting gland apparatus having a lower intrinsic capacitance is desirable because it is difficult to offset large intrinsic capacitances.